Drug addiction is the leading cause of preventable death in the United States. Effective treatments are critically needed, but developing treatments is challenging because the underlying neurobiology of addiction varies over time as the disease progresses. Our work over the past 15 years has revealed important sex and hormone effects in the different phases of addiction suggesting that its underlying neurobiology may also vary between males and females. In the last 5-yr grant period we expanded our behavioral work to include studies of the neurobiology underlying sex differences. Now, in this competitive renewal, we turn to characterization of sex-specific behavioral and neurobiological changes that underlie the development of addiction. This is important because the vast majority of data on the neurobiology of cocaine addiction is based on short access self-administration studies conducted in males. Short access conditions (1-2 hr/day), however, by virtue of their stability, do not capture critical features of addiction in humans, including an enhanced motivation to use the drug and enhanced relapse vulnerability. Extended access procedures (6-24 hr/day) have been developed that produce these characteristics and reveal that the underlying neurobiology is different from that observed under short access procedures. An enhanced motivation for cocaine, as measured under a progressive-ratio schedule, has been used to define the development of addiction in rats. This characteristic develops over an abstinence period following extended, but not short access self- administration, and develops sooner in females compared to males. We recently showed that while dopamine (DA) D1 receptor signaling in the nucleus accumbens (NAc) is a critical mediator of motivation for cocaine following short access self-administration, once addiction is established, its role is diminished and glutamate AMPA/KA receptor signaling appears to be critically involved. In females, estradiol appears to be necessary for the development of an addicted phenotype and the shift to a diminished role for DA. We hypothesize that a shift from NAc DA to AMPA transmission underlies the development of addiction, and that in females, this shift is accelerated, and estradiol is necessary for it to occur. To address this hypothesis, in Aim 1 we will determine the time-point during abstinence for this shift to determine if it underlies the development of addiction and the accelerated time-course in females. In Aim 2, we will pin-point the dopamine versus glutamate receptors involved, and in Aim 3 we will evaluate the requirement of estradiol in its development. These results will greatly contribute to our understanding of not only the neurobiological processes relevant for the development and expression of addiction, but also how sex and ovarian hormones influence these processes.